Improvements in or relating to apparatus for determining variations in width of travelling transparent birefringent film

956,636. Electric selective signalling. BRITISH CELLOPHANE Ltd. March 28,1963 [April 27, 1962], No. 16253/62. Heading G4H. Apparatus for determining variations in the width of a travelling birefringent film 2, Fig. 1, comprises two edge detecting heads 1 and 11, one for each edge of the film, and a signal integration 8 for summing signals received from both heads, each detecting head, Fig. 2, including a light source 9, a first polarising means 11, for plane polarising light emitted from the light source, a second polarising means 19 so orientated that it extinguishes light passing directly from the first polarising means, at least two photo-electric cells 13 capable of being activated by light passing through the second polarising means, and spaced apart in a direction transverse to the direction of travel of the film 2, the first and second polarising means being separated to provide a passage way through which a film edge may travel and intersect the light passing from the first polarising means to the second polarising means and each photo-electric cell being connected to the integrator so that when the cell is activated by light a signal is conveyed to the signal integration. In the embodiment shown four festoon-type lamps are used as the light source and thirty-two photo-electric cells are used, arranged in eight rows of four, transverse to the direction of travel of the film, with each row staggered (Fig. 3,) with respect to adjacent rows. Each photo-electric cell is connected between positive and negative voltage sources 21 and 22 Fig. 4, via a load resistor 23. Each load resistor controls the grid potential of a gas filled triode trigger valve 20, each valve having its cathode connected to one end of a recording ammeter 8. As the film 2 passes between the source and the cells it causes the light passing through it to be elliptically polarised, which light is then passed by the second polarising means 19 to activate a number of cells representing the degree of insertion of the film into the detector heads. Each cell so energized causes a potential rise across its load resistor 23 and trigger its associated valve 20. A current is thus passed through, and recorded by the ammeter 8, representative of the width of the film 2. Specification 627,228 is referred to.